The present invention relates to a telecommunications system and particularly but not exclusively to a digital time-division duplex radio communications system with a plurality of portable units or handsets and two or more base units forming a cordless telephone system.
Conventionally with a telecommunications system such as a CT2 wireless system which has many handsets and many base units each covering a respective cell in which one or more of the handsets may be found, the location of each of the handsets is monitored by repeated polling of the handsets at regular intervals. In this way incoming calls may be directed quickly to the correct base unit for communicating with the handset for which the call is intended. In this way the inefficiencies of multiple cell broadcast or of a handset search may be avoided. However, when the handset has been turned off or has moved out of range of the base units, typically the system searches for the handset for a given length of time and then either ceases to perform any searching for the handset or performs relatively infrequent searches. In situations such as this an incoming call would either be notified that the handset is unavailable or the call would be redirected for example to a voice mail facility.
Where continued but infrequent searches are performed by the system for the missing handset, when the handset re-enters the range of the system in due course the handset is polled in the usual manner and the usual procedure is followed enabling the handset to re-register its location. The disadvantage of this is that the handset could be within range of the system for some considerable time before it is polled and so calls may unwittingly be re-directed unnecessarily. In systems where all searching for the handset ceases after a predetermined period of time, it is necessary for the handset to be re-registered with the system before it can receive calls again. Such re-registration is usually performed either manually by the user of the handset or automatically by the handset in response to a special beacon transmission. Each procedure has disadvantages. In the case of manual re-registration the user may be unaware that the handset had been moved out of range and that re-registration is therefore necessary; where the handset performs re-registration automatically the system must reserve some of its traffic capacity for beacon purposes which is a considerable overhead where conventional tracking of the handsets is also performed.